1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to an array substrate for in-plane switching mode liquid crystal display device and a method of fabricating the same.
2. Discussion of the Related Art
In general, a liquid crystal display (LCD) device uses the optical anisotropy and polarization properties of liquid crystal molecules to produce an image. Due to the optical anisotropy of the liquid crystal molecules, refraction of light incident onto the liquid crystal molecules depends upon the alignment direction of the liquid crystal molecules. The liquid crystal molecules have long thin shapes that can be aligned along specific directions. The alignment direction of the liquid crystal molecules can be controlled by applying an electric field. Accordingly, the alignment of the liquid crystal molecules changes in accordance with the direction of the applied electric field. Thus, by properly controlling the electric field applied to a group of liquid crystal molecules within respective pixel regions, a desired image can be produced by appropriately refracting the incident light.
There are several types LCD devices, and one of which is commonly referred to as active matrix LCD (AM-LCD) device. The AM-LCD device includes an array of pixels forming a matrix. Each of the pixels in the AM-LCD device includes a thin film transistor (TFT) and a pixel electrode. The AM-LCD devices are currently being developed because of their high resolution and superior quality for displaying moving pictures.
A related art LCD device includes a color filter substrate having a common electrode, an array substrate having a pixel electrode, and a liquid crystal layer interposed between the color filter substrate and the array substrate. In the related art LCD device, the liquid crystal layer is driven by a vertical electric field between the pixel electrode and the common electrode. The related art LCD device provides a superior transmittance and a high aperture ratio. However, the related art LCD device has a narrow viewing angle because it is driven by the vertical electric field. Various other types of LCD devices having wide viewing angles, such as in-plane switching mode (IPS) mode LCD device, have been developed.
FIG. 1 is a schematic cross-sectional view of an IPS mode LCD device according to the related art. Referring to FIG. 1, an upper substrate 9 and a lower substrate 10 face and are spaced apart from each other. A liquid crystal layer 11 is interposed between the upper and the lower substrates. The upper substrate 9 and the lower substrate 10 may be commonly referred to as a color filter substrate and an array substrate, respectively. A common electrode 17 and a pixel electrode 30 are formed on the lower substrate 10. The liquid crystal layer 11 is driven by a lateral electric field “L” between the common electrode 17 and the pixel electrode 30. Since liquid crystal molecules in the liquid crystal layer 11 change directions while maintaining their longitudinal axes in a plane perpendicular to the direct viewing direction of a display, IPS provides a wide viewing angle for the display device. For example, the viewing angle can range from 80 to 85 degrees along vertical and horizontal directions from a line vertical to an IPS-LCD panel.
FIG. 2A is a schematic cross-sectional view of the related art in-plane switching mode liquid crystal display device in an ON state. Referring to FIG. 2A, voltages are applied to a pixel electrode 30 and a common electrode 17 to generate an electric field L having horizontal and vertical portions. In the vertical portion of the electric field L over the pixel electrode 30 and the common electrode 17, first liquid crystal molecules 11a of the liquid crystal layer 11 are not re-aligned the electric field L, and a phase transition of the liquid crystal layer 11 does not occur. In the horizontal portion of the electric field L between the pixel electrode 30 and the common electrode 17, second liquid crystal molecules 11b of the liquid crystal layer 11 are horizontally re-aligned with the electric field L. Thus, a phase transition of the liquid crystal layer 11 occurs in the horizontal portion of the electric field L. Because the liquid crystal molecules are re-aligned with the horizontal portion of the electric field L, the IPS mode LCD device has a wide viewing angle. For example, users can see images having a viewing angle of about 80° to about 85° along top, bottom, right and left directions with respect to a normal direction of the IPS mode LCD device.
FIG. 2B is a schematic cross-sectional view of the related art in-plane switching mode liquid crystal display device in an OFF state. Referring to FIG. 2B, an horizontal electric field is not generated when the IPS mode LCD device is in the OFF state. Thus, liquid crystal molecules 11 are not re-aligned. Thus, a phase transition of the liquid crystal layer 11 does not occur.
FIG. 3 is a plane view of the related art array substrate for an IPS mode LCD device. Referring to FIG. 3, a gate line 12 and a data line 24 crossing each other are formed on a substrate 10. A thin film transistor (TFT) Tr is disposed near each crossing of the gate line 12 and the data line 24. The TFT Tr includes a gate electrode 14, an active layer 20, a source electrode 26 and a drain electrode 28. The source electrode 26 is connected to the data line 24, and the gate electrode 14 is a portion of the gate line 12. A pixel region P is defined by the crossing of the gate line 12 and the data line 24. A plurality of pixel electrodes 30 parallel to the data line 24 is connected to the TFT Tr via a first pixel line 29a connected to the drain electrode 28. The pixel electrodes 30 are also connected to a second pixel line 29b. In addition, a plurality of common electrodes 17 extends from a common line 16 parallel to the gate line 12. The common electrodes 17 are parallel to the data line 24 and alternate with the pixel electrodes 30.
In the related art IPS mode LCD device, however, problems such as a gray inversion may occur. To improve these problems, an IPS mode LCD device having common electrodes and pixel electrodes of a chevron shape has been suggested.
FIG. 4 is a schematic plan view of the related art array substrate for an IPS mode LCD device. Referring to FIG. 4, a gate line 32 and a common line 42 are formed on a substrate 31 in parallel to each other. A data line 50 having a chevron shape crosses the gate line 32 and the common line 42 to define a pixel region. A thin film transistor (TFT) Tr is formed at the crossing of the gate line 32 and the data line 50. In addition, a plurality of common electrodes 45 having a chevron shape extends from the common line 42 in the pixel region. The common electrodes 45 are spaced apart from each other. A plurality of pixel electrodes 62 is disposed in a space between the common electrodes 45. The pixel electrodes 62 also have a chevron shape and are connected to the TFT Tr. The common electrodes 45 are combined by an auxiliary common line 43 to form a closed structure. The common line 42 and the auxiliary common line 43 adjacent to the gate line 32 function as first and second shielding means SA1 and SA2, respectively, which prevent interference with an electric field due to the gate line 32. The pixel electrodes 62 also have a closed structure.
Since the plurality of common electrodes 45 and the plurality of pixel electrodes 62 have a chevron shape, the pixel region may be divided into two portions with different directions for the electric fields generated in these portions. Accordingly, liquid crystal molecules are re-aligned along two different directions in the two portions of the pixel region, thereby forming a two-domain structure. In the two-domain structure, since birefringence is compensated in the two portions, a color shift phenomenon is minimized and an area without a gray inversion is enlarged.
However, in the related art IPS mode LCD device with the chevron-shaped common electrodes and pixel electrodes, the outermost common electrodes 45a and 45b adjacent to the data line 50 have a width cw over 10 μm to minimize a vertical cross-talk phenomenon. Further, the first and second shielding means SA1 and SA2 are required to prevent interference with an electric field due to the gate line 32. As a result, the aperture ratio is reduced.
FIG. 5 is a simulation graph showing transmittance in one pixel region of the related art IPS mode LCD device. As shown in FIG. 5, the related art IPS mode LCD device having the chevron-shaped common electrode and pixel electrodes has another disadvantage. Specifically, an effective transmittance decreases at a bent portion of a pixel electrode or a common electrode, i.e., at a borderline between two domains. Accordingly, the brightness of the IPS mode LCD device is reduced.
FIG. 6 is a schematic plane view of a data line of the related art IPS mode LCD device. Referring to FIG. 6, a data line 50 is disposed along a vertical direction. Common electrodes 45 and a pixel electrode 62 are disposed parallel to the data line 50. The common electrodes 45 are spaced apart from each other and alternate with the pixel electrode 62. An alignment layer for initial orientation of a liquid crystal layer may have an orientation direction having an angle of about 20° with respect to the vertical direction. Accordingly, a long axis of a liquid crystal molecule may be aligned to the orientation direction of the alignment layer.
When a data signal is applied to the pixel electrode 62 through the data line 50, a first electric field “E0” is generated between the pixel electrode 62 and the common electrode 45, and a second electric field “Ed” is generated between the data line 50 and the common electrode 45. Since the data line 50 and the pixel electrode 62 are parallel to the common electrode 45, the first electric field “E0” is parallel to the second electric field “Ed.” Accordingly, a total electric field “E” driving the liquid crystal layer is the summation of the first and second electric fields “E0” and “Ed”, that is, E=E0+Ed.
To minimize interference with the total electric field “E” due to the second electric field “Ed,” an outermost common electrode 45 may be formed to have a first width “W1” such that the second electric field “Ed” is sufficiently separated from the first electric field “E0.” For example, the outermost common electrode 45 may have a first thickness “W1” within a range of about 10 μm to about 15 μm.